When we are deserializing the lexical decls of a DeclContext from PCH, notify the...

When we are deserializing the lexical decls of a DeclContext from PCH, notify the PCHReader to hold off passing Decls
to the consumer until the DeclContext is fully prepared.

Before, due to recursive loading, we could be in a situation where we would try to deserialize the decls of a DeclContext which was already doing that, and bad things would happen. In the specific case I encountered, the lexical decls would
form a cycle and we would enter infinite loop territory.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@109857 91177308-0d34-0410-b5e6-96231b3b80d8